Presently available uninterrupted power supplies are based on a modular architecture whereby the uninterruptible power supply (UPS) is connected between an AC utility power line and a switch power supply (SPS). The SPS is continually fed AC power which it converts to appropriate regulated and unregulated DC output voltages, as required. Conventional design of such systems is based on a four-fold conversion of electrical energy, as follows:
(1) the incoming AC power is converted to DC at approximately the level of the backup battery voltage, with a small residual charge being available in order to ensure that the battery remains charged at all times; PA1 (2) DC to AC conversion of the DC voltage so as to provide an AC voltage level which approximates to that of the incoming AC line; PA1 (3) conversion of the AC voltage level to a high voltage DC level for energizing the high frequency converter in the SPS; and PA1 (4) multi-channel conversion of DC voltage to DC voltage so as to provide the required DC voltage outputs. PA1 the main power supply comprising:
The switch power supply receives power all the time either from the AC utility power line or, in the event of a power failure, from a backup battery. Such systems must work at relatively high power levels of typically 200-300 watts and are typically only 75-80% efficient. Moreover, the multi-stage mode of power conversion in most existing UPS systems require large and expensive components and, more particularly, duplication of regulation and switching circuits. Such duplication adds significantly to the resulting costs of the UPS.
In order to avoid such duplication, U.S. Pat. No. 5,289,045 (Lavin et al.) discloses an uninterruptible power supply which provides internal connection to an auxiliary power supply so as to provide continuous device power without duplicating regulation and switching circuits. The power supply includes an AC to DC converter, a transformer and a switch applying DC power from the converter to the transformer. A pulse width modulation control circuit assures a regulated output voltage which application of auxiliary power is controlled by a fixed time interval one-shot that assures a smooth transition to AC utility power upon resumption of such power following a failure. An auxiliary AC power output signal is maintained for a video monitor or other device even while operating on auxiliary power.
Whilst duplication of the regulation and switching circuits is indeed avoided in the UPS proposed by Lavin et at., this is at the expense of necessitating that either the main power supply and the auxiliary power supply are designed as an integral unit or, alternatively, that the main power supply be modified in order to allow effective connection thereto of the auxiliary power supply. Specifically, the main power supply proposed by Lavin et al. employs a transformer having a single primary winding and two sets of center-tapped secondary windings, each arranged as a push-pull winding pair. The primary winding is connected to a high voltage switched DC voltage whilst the first set of secondary windings is selectively wound to produce a 5 volt output, the second set of secondary windings being wound to produce plus and minus 12 volt outputs. In the event of a main power failure, the appropriate 12 volt levels are fed to the secondary windings of the transformer via a backup or auxiliary power supply energized by a pair of 12 volt batteries via a power switch. The power switch operates in response to pulse width control signals from the pulse width modulator circuit to drive alternately opposite sides of the 12 volt secondary transformer winding with the nominal 24 volt auxiliary power from the batteries.
Duplication of circuitry is avoided by using the same pulse width modulator circuit to regulate the secondary winding voltages during both battery operation and during normal AC utility power operation. However, it will be understood that this is made possible only by designing both the main power supply and the auxiliary power supply as an integral unit and this, in turn, means that the auxiliary power supply proposed by Lavin et al. is unsuitable for direct connection to conventional off-the-shelf power supplies such as are commonly used in computers and the like. It should also be noted that the solution proposed by Lavin et al. requires access to the transformer secondary windings in order to connect the auxiliary power supply thereto. This again implies redesign of the secondary transformer, thereby greatly increasing the manufacturing cost of the main power supply.
Yet a further consideration relates to the fact that the transformer 24 V secondary winding is designed for producing 12 V DC at a power rating of approximately 100 W, whilst the 12 V secondary winding is employed for producing 5 V DC at a power rating typically in the order of 200 W. When a power failure occurs in the AC power line, auxiliary power is supplied via the transformer 12 V secondary winding. The auxiliary power must be sufficient not only to produce the standard .+-.12 V DC and .+-.5 V DC levels which are produced by the main power supply, but must also provide sufficient power for allowing connection of a peripheral device, such as a video monitor, to the AC output of the power switch. Even if only the original 300 W is supplied via the transformer 12 V secondary winding, this still exceeds its power rating by a factor of 3. Bearing in mind that additional power may also be required for driving auxiliary circuitry, the total power which must be provided by the UPS may well exceed the maximum power rating of the 12 V transformer secondary winding by a factor of more than 3. Thus, the transformer 12 V secondary winding must be rewound with larger gauge wire and this requires dismantling the transformer laminations. It may then be impossible to accommodate thicker windings on the existing transformer core, requiring replacement of the whole transformer with a larger transformer having a higher power rating. A larger transformer occupies more space and usually cannot be accommodated on the original printed circuit board, requiring replacement of the complete PCB.
Furthermore, since a quasi alternating voltage is applied by the power switch to the transformer secondary winding when the main AC power fails, it is necessary to ensure that the signal supplied by the power switch is in phase with the original AC at the moment of changeover: both when the main AC power fails and also when it is restored. This requires that the power switch be synchronized by PWM control signals which are common to both the SPS and the auxiliary power supply. It is not feasible merely to hard-wire the output of the PWM in the SPS to the auxiliary power supply because of the susceptibility of stray mains frequency 60/50 Hz pickup by the wires and thus the whole of the PCB must be redesigned, thereby greatly increasing the cost of the SPS.
The manufacturing cost of a conventional power supply for use with a personal computer, for example, is of the order of US$ 8.00. To build the power supply proposed by Lavin et al. which enables connection thereto of a UPS, bearing in mind the above considerations, would increase the manufacturing cost of the main power supply by a factor of approximately 10 owing to the required redesign of the main power supply.
Even apart from cost considerations, safety factors militate against employing the auxiliary power supply proposed by Lavin et al. with a conventional power supply unless the two are co-designed as an integral UPS. Thus, the standard short circuit protection circuitry in the switch power supply cannot function to interrupt current to the load if the power switch connected to the battery backup fails, because the current supplied by the battery does not flow through the switch power supply. Consequently, a short circuit across the output of the auxiliary power supply may have irreparable and possibly dangerous consequences unless, of course, separate short circuit protection is provided within the auxiliary power supply, again adding to the overall cost.
U.S. Pat. No. 4,728,808 (Bet-Esh et al.) also discloses an uninterruptible power supply system having input terminals connectable to an AC power source and leading to an AC to DC converter for producing a first DC voltage source and a second DC voltage source operationally connected to the first source. The system supplies at the output of the second source a voltage normally primarily provided by the first DC source. A capacitive accumulator device is connected in parallel with a voltage sensing and controlling circuit and with the output of the two DC sources. The sensing and controlling circuit controls the output of at least the second of the DC voltage sources so as to provide at the output terminals of the system a substantially constant output voltage even when the AC power source to which the system is connected is interrupted.
The system disclosed by Bet-Esh et al. also requires that the UPS be designed as a composite device and does not allow connection of the second DC voltage source, constituting the backup or auxiliary DC voltage, to a conventional off-the-shelf main power supply without significant modification thereof.